Developing effective therapies for ALS patients has proven to be extraordinarily challenging. The reasons are many and complex, but dominant amongst these are the phenotypic heterogeneity of the ALS patient population and the insensitivity of clinical outcome measures to therapeutic effect during phase II clinical trials. The result is that it is very difficult to make well informed go/no-go decisions at the end of phase II with respect to which drugs to move forward into phase III clinical trials. Specific types of biomarkers are widely believed to hold great promise in helping to overcome these barriers. Prognostic biomarkers that help to predict the course of disease by adding value to what can be determined from readily available clinical parameters, might be used as eligibility or stratification criteria to ensure more homogeneous study populations within which treatment effect may be more readily demonstrable. Biomarkers whose longitudinal trajectory (and variability) is well defined have the potential to serve as pharmacodynamic biomarkers of treatment effect. Showing for example, that an experimental therapeutic leads to normalization of a biomarker level, would provide supportive evidence for the therapeutic effect of an experimental compound and help to empower decisions to advance the particular therapeutic agent into the next phase of clinical development. Intense discovery efforts over the course of the last 10+ years have yielded a plethora of biomarker candidates, but none of these have yet been validated in a multi-center fashion. In this project we aim do close this knowledge gap by undertaking a multi- center validation study of the lead biological-fluid-based biomarker candidates ? urinary p75 neurotrophin receptor extracellular domain (p75ECD), blood and cerebrospinal fluid (CSF) phosphorylated neurofilament heavy (pNfH), blood and CSF neurofilament light (NfL) and, in the population with a C9orf72 hexanucleotide repeat expansion, peripheral blood mononuclear cell (PBMC) and CSF levels of the dipeptide repeat protein poly(GP). To accomplish the proposed aims of validating these biomarkers, we will leverage the ongoing activities of the CReATe Consortium through which ~500 ALS patients will be deeply phenotyped longitudinally, undergo whole genome sequencing, and from whom a relevant set of biological fluids are being collected and stored.